Engine exhaust catalysts containing palladium-gold

ABSTRACT

An emission control catalyst that exhibits improved CO and HC reduction performance includes supported precious group metal catalysts that are coated onto different layers of the substrate for the emission control catalyst. Zeolites of one or more types are added to the emission control catalyst as a hydrocarbon absorbing component to boost the low temperature performance of the emission control catalyst. Y zeolite is used by itself or mixed with other zeolites to enhance hydrocarbon storage at low temperatures.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 12/436,028, filed May 5, 2009, now U.S. Pat. No. 7,745,367,which is a continuation of U.S. patent application Ser. No. 11/942,710,filed Nov. 20, 2007 now U.S. Pat. No. 7,534,738, which claims thebenefit of U.S. Provisional Patent Application Ser. No. 60/867,335,filed Nov. 27, 2006. U.S. patent application Ser. No. 12/436,028 isincorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments of the present invention generally relate to supportedcatalysts containing precious group metals and, and more particularly,to engine exhaust catalysts containing palladium and gold, and methodsof production thereof.

2. Description of the Related Art

Many industrial products such as fuels, lubricants, polymers, fibers,drugs, and other chemicals would not be manufacturable without the useof catalysts. Catalysts are also essential for the reduction ofpollutants, particularly air pollutants created during the production ofenergy and by automobiles. Many industrial catalysts are composed of ahigh surface area support material upon which chemically active metalnanoparticles (i.e., nanometer sized metal particles) are dispersed. Thesupport materials are generally inert, ceramic type materials havingsurface areas on the order of hundreds of square meters/gram. This highspecific surface area usually requires a complex internal pore system.The metal nanoparticles are deposited on the support and dispersedthroughout this internal pore system, and are generally between 1 and100 nanometers in size.

Processes for making supported catalysts go back many years. One suchprocess for making platinum catalysts, for example, involves thecontacting of a support material such as alumina with a metal saltsolution such as hexachloroplatinic acid in water. The metal saltsolution “impregnates” or fills the pores of the support during thisprocess. Following the impregnation, the support containing the metalsalt solution would be dried, causing the metal salt to precipitatewithin the pores. The support containing the precipitated metal saltwould then be calcined (typically in air) and, if necessary, exposed toa reducing gas environment (e.g., hydrogen or carbon monoxide) forfurther reduction to form metal particles. Another process for makingsupported catalysts involves the steps of contacting a support materialwith a metal salt solution and reducing the metal ions to metalparticles in situ using suitable reducing agents.

Supported catalysts are quite useful in removing pollutants from vehicleexhausts. Vehicle exhausts contain harmful pollutants, such as carbonmonoxide (CO), unburned hydrocarbons (HC), and nitrogen oxides (NOx),that contribute to the “smog-effect” that have plagued majormetropolitan areas across the globe. Catalytic converters containingsupported catalysts and particulate filters have been used to removesuch harmful pollutants from the vehicle exhaust. While pollution fromvehicle exhaust has decreased over the years from the use of catalyticconverters and particulate filters, research into improved supportedcatalysts has been continuing as requirements for vehicle emissioncontrol have become more stringent and as vehicle manufacturers seek touse less amounts of precious metal in the supported catalysts to reducethe total cost of emission control.

The prior art teaches the use of supported catalysts containingpalladium and gold as good partial oxidation catalysts. As such, theyhave been used extensively in the production of vinyl acetate in thevapor phase by reaction of ethylene, acetic acid and oxygen. See, e.g.,U.S. Pat. No. 6,022,823. As for vehicle emission control applications,U.S. Pat. No. 6,763,309 speculates that palladium-gold might be a goodbimetallic candidate for increasing the rate of NO decomposition. Thedisclosure, however, is based on a mathematical model and is notsupported by experimental data. There is also no teaching in this patentthat a palladium-gold system will be effective in treating vehicleemissions that include CO and HC.

SUMMARY OF THE INVENTION

The present invention provides emission control catalysts for treatingemissions that include CO and HC, and methods for producing the same.The engine may be a vehicle engine, an industrial engine, or generally,any type of engine that burns hydrocarbons.

An emission control catalyst according to embodiments of the presentinvention exhibits improved CO and HC reduction performance. Theemission control catalyst includes supported precious group metalcatalysts that are coated onto different layers of the substrate for theemission control catalyst. Zeolites of one or more types are added tothe emission control catalyst as a hydrocarbon absorbing component toboost the low temperature performance of the emission control catalyst.The zeolites absorb hydrocarbons in the exhaust gas and store them atthe cold start stage when the catalyst temperature is relatively low.After the catalyst temperature rises up, both the stored hydrocarbonsand other pollutants in the exhaust gas are converted. Y zeolite is usedby itself or mixed with other zeolites in one or more embodiments of theinvention to enhance hydrocarbon storage at low temperatures.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features of the presentinvention can be understood in detail, a more particular description ofthe invention, briefly summarized above, may be had by reference toembodiments, some of which are illustrated in the appended drawings. Itis to be noted, however, that the appended drawings illustrate onlytypical embodiments of this invention and are therefore not to beconsidered limiting of its scope, for the invention may admit to otherequally effective embodiments.

FIGS. 1A-1D are schematic representations of different engine exhaustsystems in which embodiments of the present invention may be used.

FIG. 2 is an illustration of a catalytic converter with a cut-awaysection that shows a substrate onto which emission control catalystsaccording to embodiments of the present invention are coated.

FIGS. 3A-3D illustrate different configurations of a substrate for anemission control catalyst.

FIG. 4 is a flow diagram illustrating the steps for preparing anemission control catalyst according to an embodiment of the presentinvention.

FIG. 5 is a flow diagram illustrating the steps for preparing anemission control catalyst according to another embodiment of the presentinvention.

FIG. 6 is a bar graph illustrating the improved conversion efficiency ofan emission control catalyst according to an embodiment of the presentinvention.

DETAILED DESCRIPTION

In the following, reference is made to embodiments of the invention.However, it should be understood that the invention is not limited tospecific described embodiments. Instead, any combination of thefollowing features and elements, whether related to differentembodiments or not, is contemplated to implement and practice theinvention. Furthermore, in various embodiments the invention providesnumerous advantages over the prior art. However, although embodiments ofthe invention may achieve advantages over other possible solutionsand/or over the prior art, whether or not a particular advantage isachieved by a given embodiment is not limiting of the invention. Thus,the following aspects, features, embodiments and advantages are merelyillustrative and are not considered elements or limitations of theappended claims except where explicitly recited in the claims. Likewise,reference to “the invention” shall not be construed as a generalizationof any inventive subject matter disclosed herein and shall not beconsidered to be an element or limitation of the appended claims exceptwhere explicitly recited in the claims.

FIGS. 1A-1D are schematic representations of different engine exhaustsystems in which embodiments of the present invention may be used. Thecombustion process that occurs in an engine 102 produces harmfulpollutants, such as CO, various hydrocarbons, particulate matter, andnitrogen oxides (NOx), in an exhaust stream that is discharged through atail pipe 108 of the exhaust system.

In the exhaust system of FIG. 1A, the exhaust stream from an engine 102passes through a catalytic converter 104, before it is discharged intothe atmosphere (environment) through a tail pipe 108. The catalyticconverter 104 contains supported catalysts coated on a monolithicsubstrate that treat the exhaust stream from the engine 102. The exhauststream is treated by way of various catalytic reactions that occurwithin the catalytic converter 104. These reactions include theoxidation of CO to form CO₂, burning of hydrocarbons, and the conversionof NO to NO₂.

In the exhaust system of FIG. 1B, the exhaust stream from the engine 102passes through a catalytic converter 104 and a particulate filter 106,before it is discharged into the atmosphere through a tail pipe 108. Thecatalytic converter 104 operates in the same manner as in the exhaustsystem of FIG. 1A. The particulate filter 106 traps particulate matterthat is in the exhaust stream, e.g., soot, liquid hydrocarbons,generally particulates in liquid form. In an optional configuration, theparticulate filter 106 includes a supported catalyst coated thereon forthe oxidation of NO and/or to aid in combustion of particulate matter.

In the exhaust system of FIG. 1C, the exhaust stream from the engine 102passes through a catalytic converter 104, a pre-filter catalyst 105 anda particulate filter 106, before it is discharged into the atmospherethrough a tail pipe 108. The catalytic converter 104 operates in thesame manner as in the exhaust system of FIG. 1A. The pre-filter catalyst105 includes a monolithic substrate and supported catalysts coated onthe monolithic substrate for the oxidation of NO. The particulate filter106 traps particulate matter that is in the exhaust stream, e.g., soot,liquid hydrocarbons, generally particulates in liquid form.

In the exhaust system of FIG. 1D, the exhaust stream passes from theengine 102 through a catalytic converter 104, a particulate filter 106,a selective catalytic reduction (SCR) unit 107 and an ammonia slipcatalyst 110, before it is discharged into the atmosphere through a tailpipe 108. The catalytic converter 104 operates in the same manner as inthe exhaust system of FIG. 1A. The particulate filter 106 trapsparticulate matter that is in the exhaust stream, e.g., soot, liquidhydrocarbons, generally particulates in liquid form. In an optionalconfiguration, the particulate filter 106 includes a supported catalystcoated thereon for the oxidation of NO and/or to aid in combustion ofparticulate matter. The SCR unit 107 is provided to reduce the NOxspecies to N₂. The SCR unit 107 may be ammonia/urea based or hydrocarbonbased. The ammonia slip catalyst 110 is provided to reduce the amount ofammonia emissions through the tail pipe 108. An alternativeconfiguration places the SCR unit 107 in front of the particulate filter106.

Alternative configurations of the exhaust system includes the provisionof SCR unit 107 and the ammonia slip catalyst 110 in the exhaust systemof FIG. 1A or 1C, and the provision of just the SCR unit 107, withoutthe ammonia slip catalyst 110, in the exhaust system of FIG. 1A, 1B or1C.

As particulates get trapped in the particulate filter within the exhaustsystem of FIG. 1B, 1C or 1D, it becomes less effective and regenerationof the particulate filter becomes necessary. The regeneration of theparticulate filter can be either passive or active. Passive regenerationoccurs automatically in the presence of NO₂. Thus, as the exhaust streamcontaining NO₂ passes through the particulate filter, passiveregeneration occurs. During regeneration, the particulates get oxidizedand NO₂ gets converted back to NO. In general, higher amounts of NO₂improve the regeneration performance, and thus this process is commonlyreferred to as NO₂ assisted oxidation. However, too much NO₂ is notdesirable because excess NO₂ is released into the atmosphere and NO₂ isconsidered to be a more harmful pollutant than NO. The NO₂ used forregeneration can be formed in the engine during combustion, from NOoxidation in the catalytic converter 104, from NO oxidation in thepre-filter catalyst 105, and/or from NO oxidation in a catalyzed versionof the particulate filter 106.

Active regeneration is carried out by heating up the particulate filter106 and oxidizing the particulates. At higher temperatures, NO₂assistance of the particulate oxidation becomes less important. Theheating of the particulate filter 106 may be carried out in various waysknown in the art. One way is to employ a fuel burner which heats theparticulate filter 106 to particulate combustion temperatures. Anotherway is to increase the temperature of the exhaust stream by modifyingthe engine output when the particulate filter load reaches apre-determined level.

The present invention provides catalysts that are to be used in thecatalytic converter 104 shown in FIGS. 1A-1D, or generally as catalystsin any vehicle emission control system, including as a diesel oxidationcatalyst, a diesel filter catalyst, an ammonia-slip catalyst, an SCRcatalyst, or as a component of a three-way catalyst. The presentinvention further provides a vehicle emission control system, such asthe ones shown in FIGS. 1A-1D, comprising an emission control catalystcomprising a monolith and a supported catalyst coated on the monolith.

FIG. 2 is an illustration of a catalytic converter with a cut-awaysection that shows a substrate 210 onto which supported metal catalystsare coated. The exploded view of the substrate 210 shows that thesubstrate 210 has a honeycomb structure comprising a plurality ofchannels into which washcoats containing supported metal catalysts areflowed in slurry form so as to form coating 220 on the substrate 210.

FIGS. 3A-3D illustrate different embodiments of the present invention.In the embodiment of FIG. 3A, coating 220 comprises two washcoat layers221, 223 on top of substrate 210. Washcoat layer 221 is the bottom layerthat is disposed directly on top of the substrate 210 and contains metalparticles having palladium and gold in close contact (also referred toas “palladium-gold metal particles”). Washcoat layer 223 is the toplayer that is in direct contact with the exhaust stream and containsmetal particles having platinum alone or in close contact with anothermetal species such as palladium (also referred to as“platinum-containing metal particles”). Based on their positionsrelative to the exhaust stream, washcoat layer 223 encounters theexhaust stream before washcoat layer 221.

In the embodiment of FIG. 3B, coating 220 comprises three washcoatlayers 221, 222, 223 on top of substrate 210. Washcoat layer 221 is thebottom layer that is disposed directly on top of the substrate 210 andincludes palladium-gold metal particles. Washcoat layer 223 is the toplayer that is in direct contact with the exhaust stream and includesplatinum-containing metal particles. Washcoat layer 222 is the middlelayer that is disposed in between washcoat layers 221, 223. The middlelayer is provided to minimize the interaction between the Pt and Pd—Aucomponents. The middle layer may be a blank support or may containzeolites, rare earth oxides, or inorganic oxides. Based on theirpositions relative to the exhaust stream, washcoat layer 223 encountersthe exhaust stream before washcoat layers 221, 222, and washcoat layer222 encounters the exhaust stream before washcoat layer 221.

In the embodiment of FIG. 3C, the substrate 210 is a single monoliththat has two coating zones 210A, 210B. A washcoat includingplatinum-containing metal particles is coated onto a first zone 210A anda washcoat including palladium-gold metal particles is coated onto asecond zone 210B.

In the embodiment of FIG. 3D, the substrate 210 includes first andsecond monoliths 231, 232, which are physically separate monoliths. Awashcoat including platinum-containing metal particles is coated ontothe first monolith 231 and a washcoat including palladium-gold metalparticles is coated onto the second monolith 232.

All of the embodiments described above include a palladium-gold catalystin combination with a platinum-based catalyst. The weight ratio ofpalladium to gold in the palladium-gold catalyst is about 0.05:1 to20:1, preferably from about 0.5:1 to about 2:1. The palladium-goldcatalyst may be promoted with bismuth or other known promoters. Theplatinum-based catalyst may be a platinum catalyst, a platinum-palladiumcatalyst, a platinum catalyst promoted with bismuth or other nowpromoters, or other platinum-based catalysts (e.g., Pt—Rh, Pt—Ir, Pt—Ru,Pt—Au, Pt—Ag, Pt—Rh—Ir, Pt—Ir—Au, etc.). The preferred embodimentsemploy a platinum-palladium catalyst as the platinum-based catalyst. Theweight ratio of platinum to palladium in this catalyst is about 0.05:1to 20:1, preferably from about 2:1 to about 4:1.

In addition, the platinum-based catalyst is situated so that itencounters the exhaust stream prior to the palladium-gold catalyst. Bypositioning the platinum-based catalyst relative to the palladium-goldcatalyst in this manner, the inventors have discovered that HCinhibition effects on the oxidation activity of the palladium-goldcatalyst are reduced to sufficient levels so that the overall catalyticperformance is improved. In the embodiments of FIGS. 3A and 3B, theplatinum-based catalyst is included in the top layer 223 and thepalladium-gold catalyst is included in the bottom layer 221. In theembodiment of FIG. 3C, the platinum-based catalyst is included in thefirst zone 210A and the palladium-gold catalyst is included in thesecond zone 210B. In the embodiment of FIG. 3D, the platinum-basedcatalyst is included in the first monolith 231 and the palladium-goldcatalyst is included in the second monolith 232.

In additional embodiments of the present invention, a hydrocarbonabsorbing material is added to the emission control catalyst.Preferably, the hydrocarbon absorbing material is added to the emissioncontrol catalyst so that it encounters exhaust stream prior to thepalladium-gold catalyst. By positioning the hydrocarbon absorbingmaterial relative to the palladium-gold catalyst in this manner, theinventors have discovered that HC inhibition effects on the oxidationactivity of the palladium-gold catalyst are reduced to sufficient levelsso that the overall catalytic performance is improved. In theconfiguration shown in FIG. 3A, the hydrocarbon absorbing material maybe included in the top layer 223. In the configuration shown in FIG. 3B,the hydrocarbon absorbing material may be included in the middle layer222 or the top layer 223. In the configuration shown in FIG. 3C, thehydrocarbon absorbing material may be included in the first zone 210A.In the configuration shown in FIG. 3D, the hydrocarbon absorbingmaterial may be included in the front monolith 231.

In other embodiments of the present invention, any of the washcoatlayers or zones, or monoliths may include rare-earth oxides, such ascerium(IV) oxide (CeO₂) and ceria-zirconia (CeO₂—ZrO₂).

FIG. 4 is a flow diagram that illustrates the steps for preparing anemission control catalyst according to an embodiment of the presentinvention using the substrate 210. In step 410, a first supportedcatalyst, e.g., supported palladium-gold catalyst, is prepared isaccordance with known methods or with the methods described in theexamples provided below. In step 412, a second supported catalyst, e.g.,supported platinum-based catalyst, is prepared in accordance with knownmethods or with the methods described in the examples provided below. Amonolithic substrate, such as substrate 210 shown in FIG. 2 (ormonolithic substrates 231, 232 shown in FIG. 3D) is provided in step414. Exemplary monolithic substrates include those that are ceramic(e.g., cordierite), metallic, or silicon carbide based. In step 416, thefirst supported catalyst in powder form are mixed in a solvent to form awashcoat slurry, and the washcoat slurry is coated as the bottom layerof the substrate 210 or onto a rear zone or rear monolith of thesubstrate 210. In step 418, the second supported catalyst in powder formare mixed in a solvent to form a washcoat slurry, and the washcoatslurry is coated as the top layer of the substrate 210 or onto a frontzone or front monolith of the substrate 210. Optionally, zeolite orzeolite mixture including one or more of beta zeolite, ZSM-5 zeolite,and other types of zeolites is added to the washcoat slurry before thewashcoat slurry is coated in step 418.

FIG. 5 is a flow diagram that illustrates the steps for preparing anemission control catalyst according to another embodiment of the presentinvention using the substrate 210. In step 510, a first supportedcatalyst, e.g., supported palladium-gold catalyst, is prepared isaccordance with known methods or with the methods described in theexamples provided below. In step 512, a second supported catalyst, e.g.,supported platinum-based catalyst, is prepared in accordance with knownmethods or with the methods described in the examples provided below. Amonolithic substrate, such as substrate 210 shown in FIG. 2, is providedin step 514. Exemplary monolithic substrates include those that areceramic (e.g., cordierite), metallic, or silicon carbide based. In step516, the first supported catalyst in powder form are mixed in a solventto form a washcoat slurry, and the washcoat slurry is coated as thebottom layer of the substrate 210. In step 517, zeolite or zeolitemixture is added to a solvent to form a washcoat slurry and thiswashcoat slurry is coated as the middle layer of the substrate 210. Instep 518, the second supported catalyst in powder form are mixed in asolvent to form a washcoat slurry, and the washcoat slurry is coated asthe top layer of the substrate 210.

In the examples provided below, zeolite is employed as a hydrocarbonabsorbing material in the middle layer of a three-layered system shownin FIG. 3B. Zeolite is crystalline aluminosilicate. There are more than150 types of structures of zeolite, and each structure consists of welldefined pores with size in the range of 0.3 nm to 2.0 nm. In principle,only gas molecules with sizes smaller than the pores can be absorbed andstored in zeolites. The largest amount of hydrocarbon is absorbed whenthe pore size of zeolite is similar to the size of the hydrocarbonmolecules.

The Si:Al ratio of the zeolite is another factor that influences thehydrocarbon storage effect. Zeolites with high Si:Al ratio can typicallystore more hydrocarbons and have better thermal stability. Therefore,zeolites with a high Si:Al ratio, e.g., greater than 5, are preferred.Zeolites with a Si:Al ratio of greater than 20 are even more preferred.

Zeolites used in one or more embodiments of the invention may be a betazeolite, ZSM-5 zeolite, Y zeolite, SSZ-33 zeolite, mordenite, andcombinations of the foregoing in any weight ratio. Because thehydrocarbon components in a vehicle exhaust are made up of small and bigmolecules, combinations of zeolites with different pore sizes areemployed to enhance the hydrocarbon storage effect.

Y zeolite is employed in the middle layer of Examples 1 and 2 and the COand hydrocarbon conversion performance of these Examples is comparedagainst CO and hydrocarbon (in particular propene and xylene) conversionperformance of a comparable catalyst with the same amount of zeoliteloading but without Y zeolite (Example 3). The composition of Examples1-3 is provided below in Table 1.

TABLE 1 Middle Layer ZSM-5 beta Y Bottom Catalyst Top Layer (g/in³)(g/in³) (g/in³) Layer Example 1 Pt:Pd wt ratio 0.1 0 0.2 Pd:Au Example 2is 3.0:1.5 0 0 0.3 wt ratio Example 3 total loading = 0.15 0.15 0 is1.7:2.0 (comparative) 0.8 g/in³ total loading = 1.5 g/in³

The CO/propene/xylene conversion performance of Examples 1-3 is comparedby measuring the T20, T50, and T70 temperatures. The T20/T50/T70temperatures correspond to the temperatures at which 20%/50%/70% ofCO/propene/xylene will be converted (i.e., oxidized or burned).Generally, higher conversion is observed at higher temperatures andlower conversion is observed at lower temperatures.

The T20, T50 and T70 temperatures of catalysts differ depending on theconditions under which the conversion is observed. Therefore, they aredetermined under conditions that simulate the actual operatingconditions of the catalyst as closely as possible. Examples 1-3 areuseful as diesel exhaust catalysts, and thus their T20, T50, T70temperatures have been determined under simulated diesel exhaustconditions, which were as follows. A gas mixture having the composition:CO (1000 ppm), CO₂ (10%), O₂ (20%), H₂O (2%), C₃H₆ (230 ppm), C₃H₈ (110ppm), xylene (120 ppm), NO (150 ppm), and He (balance), is supplied intoa fixed bed flow reactor containing the example catalysts at a totalflow rate of 3240 ml/min. The reactor is heated from 40° C. to 450° C.at 10° C./minute. As the reactor is heated, CO/propene/xylene conversionwas measured by use of mass spectrometry as a function of temperature.The T20, T50 and T70 temperatures of Examples 1-3 for converting CO,propene and xylene are provided below in Table 2. As shown in Table 2,Examples 1 and 2 convert each of CO, propene and xylene at lowertemperatures than Example 3. It is expected that the presence of Yzeolite in Examples 1 and 2 helps with the improved conversionefficiency of these catalysts.

TABLE 2 CO Propene Xylene T20 T50 T70 T20 T50 T70 T20 T50 T70 Catalyst(° C.) (° C.) (° C.) (° C.) (° C.) (° C.) (° C.) (° C.) (° C.) Example 1156 171 176 179 187 190 180 186 189 Example 2 146 160 167 171 179 184172 179 185 Example 3 163 178 183 190 196 198 189 193 197 (comparative)

The hydrocarbon storage capacity of each of Examples 1-3 has also beenmeasured using TPD (temperature programmed desorption) experiments andcompared. The procedure for the experiment was as follows: (1) Flowhelium through example catalyst (i.e., one of Examples 1-3). (2) Exposeexample catalyst to specific adsorbate gases at room temperature (30°C.) for some time until all possible adsorption sites are saturated withhydrocarbons. (3) Flow helium through example catalyst at roomtemperature to remove the physisorbed hydrocarbons. (4) Flow heliumthrough example catalyst and ramp up the temperature to 450° C. (5) Cooldown example catalyst. The following data were monitored using a massspectrometer: the total adsorption amount (Atot), the desorption amountat room temperature (Drt), and desorption amount at high temperature(T>150° C.) (Dht). In general, it is desirable to have high Atot, highDht, and low Drt. The monitored data for each of xylene and decane aretabulated below in Table 3. As shown, Examples 1 and 2 perform betteroverall in storing xylene and decane than Example 3. Again, it isexpected that the presence of Y zeolite in Examples 1 and 2 helps withthe improved hydrocarbon storage capacity.

TABLE 3 Xylene Decane Atot Drt Dht Atot Drt Dht Catalyst (mol/g) (mol/g)(mol/g) (mol/g) (mol/g) (mol/g) Example 1 1.68e−04 7.85e−05 4.28e−061.26e−04 5.48e−05 2.67e−05 Example 2 2.07e−04 7.19e−05 1.65e−05 1.50e−045.80e−05 4.82e−05 Example 3 5.42e−05 3.79e−05 1.45e−06 7.92e−05 4.78e−059.42e−06 (comparative)

Vehicle tests were carried out to observe the effect of Y zeolite on COand hydrocarbon conversion efficiency. The formulation of the catalystcontaining Y zeolite (Example 4) and the catalyst containing ZSM-5zeolite and beta zeolite (Example 5) is shown in Table 4. Prior tovehicle testing, Examples 4 and 5 were hydrothermally aged at 840° C.for 10 hours. Two vehicle tests were carried out on each examplecatalyst. The CO and HC conversion efficiency of Examples 4 and 5 duringthe vehicles tests is shown in FIG. 6. It can be seen from FIG. 6 thatExample 4 performs better than Example 5.

TABLE 4 Middle Layer ZSM-5 beta Y Bottom Catalyst Top Layer (g/in³)(g/in³) (g/in³) Layer Example 4 Pt:Pd wt ratio 0.15 0.15 0.15 Pd:AuExample 5 is 4.0:2.0 0.15 0.15 0 wt ratio (comparative) total loading =is 3.0:3.6 0.9 g/in³ total loading = 1.0 g/in³

The preparation methods for Examples 1-5 were as follows:

Preparation of a 1.7% Pd, 2.0% Au Supported PdAu Catalyst.

Lanthanum-stabilized alumina (578 g, having a surface area of ˜200 m²/g)and 2940 mL of de-ionized water (>18 MΩ) were added to a 5 L plasticbeaker and magnetically stirred at about 500 rpm. The pH measured was8.5 and the temperature measured was 25° C. After 20 minutes, Pd(NO₃)₂(67.8 g of 14.8% aqueous solution) was gradually added over a period of10 min. The pH measured was 4.3. After stirring for 20 minutes, a secondmetal, HAuCl₄.3H₂O (24 g dissolved in 50 mL of de-ionized water), wasadded over a period of 5 min. The pH was 4.0 and the temperature of themetal-support slurry was 25° C. The metal-support slurry was stirred foran additional 30 min. In a second vessel, NaBH₄ (29.4 g) and NaOH (31.1g) were added to N₂H₄ (142 mL of 35% aqueous solution) and stirred untilthe mixture became clear. This mixture constituted the reducing agentmixture. The metal-support slurry and reducing agent mixture werecombined continuously using two peristaltic pumps. The two streams werecombined using a Y joint connected to a Vigreux column to causeturbulent mixing. The reaction product leaving the mixing chamber, i.e.,the Vigreux column, was pumped into an intermediate vessel of smallervolume and continuously stirred. The product in the intermediate vesselwas continuously pumped into a larger vessel, i.e., 5 L beaker, forresidence and with continued stirring. The entire addition/mixingprocess lasted about 30 min. The resulting product slurry was stirred inthe larger vessel for an additional period of 1 hour. The final pH was11.0 and the temperature was 25° C. The product slurry was then filteredusing vacuum techniques via Buchner funnels provided with a double layerof filter paper having 3 μm porosity. The filter cake was then washedwith about 20 L of de-ionized water in several approximately equalportions. Thereafter, the washed cake was dried at 110° C., ground to afine powder using a mortar and pestle, and subsequently calcined at 500°C. for 2 hours, with a heating rate of 8° C./min. This supported PdAucatalyst powder (1.7% Pd, 2.0% Au) was used in preparing Examples 1-3.

Preparation of a 3.0% Pd, 3.6% Au Supported PdAu Catalyst.

Lanthanum-stabilized alumina (600 g, having a surface area of ˜200 m²/g)and 3051 mL of de-ionized water (>18 MΩ) were added to a 5 L plasticbeaker and magnetically stirred at about 500 rpm. The pH measured was8.5 and the temperature measured was 25° C. After 20 minutes, Pd(NO₃)₂(130.2 g of 14.8% aqueous solution) was gradually added over a period of10 min. The pH measured was 4.3. After stirring for 20 minutes, a secondmetal, HAuCl₄.3H₂O (46.2 g dissolved in 90 mL of de-ionized water), wasadded over a period of 5 min. The pH was 4.0 and the temperature of themetal-support slurry was 25° C. The metal-support slurry was stirred foran additional 30 min. In a second vessel, NaBH4 (56.7 g) and NaOH (59.6g) were added to N₂H₄ (273 mL of 35% aqueous solution) and stirred untilthe mixture became clear. This mixture constituted the reducing agentmixture. The metal-support slurry and reducing agent mixture werecombined continuously using two peristaltic pumps. The two streams werecombined using a Y joint connected to a Vigreux column to causeturbulent mixing. The reaction product leaving the mixing chamber, i.e.,the Vigreux column, was pumped into an intermediate vessel of smallervolume and continuously stirred. The product in the intermediate vesselwas continuously pumped into a larger vessel, i.e., 5 L beaker, forresidence and with continued stirring. The entire addition/mixingprocess lasted about 30 min. The resulting product slurry was stirred inthe larger vessel for an additional period of 1 hour. The final pH was11.0 and the temperature was 25° C. The product slurry was then filteredusing vacuum techniques via Buchner funnels provided with a double layerof filter paper having 3 μm porosity. The filter cake was then washedwith about 20 L of de-ionized water in several approximately equalportions. Thereafter, the washed cake was dried at 110° C., ground to afine powder using a mortar and pestle, and subsequently calcined at 500°C. for 2 hours, with a heating rate of 8° C./min. This supported PdAucatalyst powder (3.0% Pd, 3.6% Au) was used in preparing Examples 4 and5.

Preparation of a 3.0% Pt, 1.5% Pd Supported Catalyst.

To 10 L of de-ionized H₂O was added 1940 g of La-stabilized alumina(having a BET surface area of ˜200 m²/g) followed by stirring for 30minutes at room temperature. To this slurry was added 490.6 g ofPt(NO₃)₂ solution (12.23% Pt(NO₃)₂ by weight), followed by stirring atroom temperature for 60 minutes. Acrylic acid (750 mL, 99% purity) wasthen added into the system over 12 minutes and the resulting mixture wasallowed to continue stirring at room temperature for 2 hours. The solidLa-doped alumina supported Pt catalyst was separated from the liquid viafiltration, dried at 120° C. for 2 hours, ground into a fine powder, andcalcined in air for 2 hours at a temperature of 500° C. (heated at 8°C./min) to give a 3% Pt material.

To 9.25 L of de-ionized H₂O was added 1822 g of the above 3% Pt materialfollowed by stirring for 20 minutes at room temperature. To this slurrywas added 194.4 g of Pd(NO₃)₂ solution (14.28% Pd(NO₃)₂ by weight),followed by stirring at room temperature for 60 minutes. An aqueousascorbic acid solution (930 g in 4.5 L of de-ionized H₂O) was then addedover 25 minutes followed by stirring for 60 minutes. The solid La-dopedalumina supported PtPd catalyst was separated from the liquid viafiltration, dried at 120° C. for 2 hours, ground into a fine powder, andcalcined in air for 2 hours at a temperature of 500° C. (heated at 8°C./min) to give a 3% Pt, 1.5% Pd material. This material was used inpreparing Examples 1-3.

Preparation of a 4.0% Pt, 2.0% Pd Supported Catalyst.

To 10 L of de-ionized H₂O was added 2000 g of La-stabilized alumina(having a BET surface area of ˜200 m²/g) followed by stirring for 30minutes at room temperature. To this slurry was added 695.9 g ofPt(NO₃)₂ solution (12.23% Pt(NO₃)₂ by weight), followed by stirring atroom temperature for 60 minutes. Acrylic acid (1047 mL, 99% purity) wasthen added into the system over 12 minutes and the resulting mixture wasallowed to continue stirring at room temperature for 2 hours. The solidLa-doped alumina supported Pt catalyst was separated from the liquid viafiltration, dried at 120° C. for 2 hours, ground into a fine powder, andcalcined in air for 2 hours at a temperature of 500° C. (heated at 8°C./min) to give a 4% Pt material.

To 9.25 L of de-ionized H₂O was added 1822 g of the above 4% Pt materialfollowed by stirring for 20 minutes at room temperature. To this slurrywas added 260.4 g of Pd(NO₃)₂ solution (14.28% Pd(NO₃)₂ by weight),followed by stirring at room temperature for 60 minutes. An aqueousascorbic acid solution (1230 g in 4.5 L of de-ionized H₂O) was thenadded over 25 minutes followed by stirring for 60 minutes. The solidLa-doped alumina supported PtPd catalyst was separated from the liquidvia filtration, dried at 120° C. for 2 hours, ground into a fine powder,and calcined in air for 2 hours at a temperature of 500° C. (heated at8° C./min). This supported PtPd catalyst powder (4.0% Pt, 2.0% Pd) wasused in preparing Examples 4 and 5.

EXAMPLE 1 PdAu (at 1.5 g/in³) 1st Layer, Zeolite Mixture (ZSM/Y=0.1/0.2g/in³) 2nd Layer, PtPd (at 0.8 g/in³) 3rd Layer.

The supported PdAu catalyst powder (1.7% Pd, 2.0% Au) prepared asdescribed above was made into a washcoat slurry via addition tode-ionized water, milling to an appropriate particle size (typicallywith a d50 range from 3 to 7 μm), and pH adjustment to give anappropriate viscosity for washcoating. According to methods known in theart, the washcoat slurry was coated onto a round cordierite monolith(Corning, 400 cpsi, 54.63 in³), dried at 120° C. and calcined at 500° C.to give the first layer of the multi-layer coated monolith, such thatthe PdAu loading was ˜1.5 g/in³.

Then, ZSM-5 zeolite and Y zeolite with a weight ratio of 1:2 werecombined and made into a washcoat slurry via addition to de-ionizedwater, milling to an appropriate particle size (typically with a d50range from 3 to 7 μm), and pH adjustment to give an appropriateviscosity for washcoating. According to methods known in the art, thezeolite washcoat slurry was coated onto the cordierite monolith (withthe first layer of PtPd), dried at 120° C. and calcined at 500° C. togive the second layer of the multi-layer coated monolith. The ZSM-5 andY zeolite washcoat loading are 0.1 g/in³ and 0.2 g/in³, respectively.

Then, the supported PtPd catalyst powder (3.0% Pt, 1.5% Pd) prepared asdescribed above was made into a washcoat slurry via addition tode-ionized water, milling to an appropriate particle size (typicallywith a d50 range from 3 to 7 μm), and pH adjustment to give anappropriate viscosity for washcoating. According to methods known in theart, the washcoat slurry was coated onto the cordierite monolith (withthe first layer of PdAu and the second layer of zeolite mixtures), driedat 120° C. and calcined at 500° C. to give the third layer of themulti-layer coated monolith, such that the PtPd loading was ˜0.8 g/in³.

The multi-layer coated monolith was canned according to methods known inthe art and tested using a certified testing facility on a light-dutydiesel vehicle, as described above.

EXAMPLE 2 PdAu (at 1.5 g/in³) 1st Layer, Zeolite (Y=0.3 g/in³) 2ndLayer, PtPd (at 0.8 g/in³) 3rd Layer.

The supported PdAu catalyst powder (1.7% Pd, 2.0% Au) prepared asdescribed above was made into a washcoat slurry via addition tode-ionized water, milling to an appropriate particle size (typicallywith a d50 range from 3 to 7 μm), and pH adjustment to give anappropriate viscosity for washcoating. According to methods known in theart, the washcoat slurry was coated onto a round cordierite monolith(Corning, 400 cpsi, 54.63 in³), dried at 120° C. and calcined at 500° C.to give the first layer of the multi-layer coated monolith, such thatthe PdAu loading was ˜1.5 g/in³.

Then, Y zeolite was made into a washcoat slurry via addition tode-ionized water, milling to an appropriate particle size (typicallywith a d50 range from 3 to 7 μm), and pH adjustment to give anappropriate viscosity for washcoating. According to methods known in theart, the zeolite washcoat slurry was coated onto the cordierite monolith(with the first layer of PtPd), dried at 120° C. and calcinated at 500°C. to give the second layer of the multi-layer coated monolith. The Yzeolite washcoat loading is 0.3 g/in³.

Then, the supported PtPd catalyst powder (3.0% Pt, 1.5% Pd) prepared asdescribed above was made into a washcoat slurry via addition tode-ionized water, milling to an appropriate particle size (typicallywith a d50 range from 3 to 7 μm), and pH adjustment to give anappropriate viscosity for washcoating. According to methods known in theart, the washcoat slurry was coated onto the cordierite monolith (withthe first layer of PdAu and the second layer of zeolite), dried at 120°C. and calcined at 500° C. to give the third layer of the multi-layercoated monolith, such that the PtPd loading was ˜0.8 g/in³.

The multi-layer coated monolith was canned according to methods known inthe art and tested using a certified testing facility on a light-dutydiesel vehicle, as described above.

EXAMPLE 3 PdAu (at 1.5 g/in³) 1st Layer, Zeolite Mixture(ZSM/β=0.15/0.0.15 g/in³3) 2nd Layer, PtPd (at 0.8 g/in³) 3rd Layer.

The supported PdAu catalyst powder (1.7% Pd, 2.0% Au) prepared asdescribed above was made into a washcoat slurry via addition tode-ionized water, milling to an appropriate particle size (typicallywith a d50 range from 3 to 7 μm), and pH adjustment to give anappropriate viscosity for washcoating. According to methods known in theart, the washcoat slurry was coated onto a round cordierite monolith(Corning, 400 cpsi, 54.63 in³), dried at 120° C. and calcined at 500° C.to give the first layer of the multi-layer coated monolith, such thatthe PdAu loading was ˜1.5 g/in³.

Then, ZSM-5 zeolite and beta zeolite with a weight ratio of 1:1 werecombined and made into a washcoat slurry via addition to de-ionizedwater, milling to an appropriate particle size (typically with a d50range from 3 to 7 μm), and pH adjustment to give an appropriateviscosity for washcoating. According to methods known in the art, thezeolite washcoat slurry was coated onto the cordierite monolith (withthe first layer of PtPd), dried at 120° C. and calcinated at 500° C. togive the second layer of the multi-layer coated monolith. Both ZSM-5zeolite and beta zeolite washcoat loading are 0.15 g/in³.

Then, the supported PtPd catalyst powder (3.0% Pt, 1.5% Pd) prepared asdescribed above was made into a washcoat slurry via addition tode-ionized water, milling to an appropriate particle size (typicallywith a d50 range from 3 to 7 μm), and pH adjustment to give anappropriate viscosity for washcoating. According to methods known in theart, the washcoat slurry was coated onto the cordierite monolith (withthe first layer of PdAu and the second layer of zeolite mixtures), driedat 120° C. and calcined at 500° C. to give the third layer of themulti-layer coated monolith, such that the PtPd loading was ˜0.8 g/in³.

The multi-layer coated monolith was canned according to methods known inthe art and tested using a certified testing facility on a light-dutydiesel vehicle, as described above.

EXAMPLE 4 PdAu (at 1.0 g/in³) 1st Layer, Zeolite Mixture(ZSM/β/Y=0.15/0.15/0.15 g/in³) 2nd Layer, PtPd (at 0.9 g/in³) 3rd Layer.

The supported PdAu catalyst powder (3.0% Pd, 3.6% Au) prepared asdescribed above was made into a washcoat slurry via addition tode-ionized water, milling to an appropriate particle size (typicallywith a d50 range from 3 to 7 μm), and pH adjustment to give anappropriate viscosity for washcoating. According to methods known in theart, the washcoat slurry was coated onto a round cordierite monolith(Corning, 350 cpsi, 48.16 in³), dried at 120° C. and calcined at 500° C.to give the first layer of the multi-layer coated monolith, such thatthe PdAu loading was ˜1.0 g/in³.

Then, ZSM-5 zeolite, beta zeolite, and Y zeolite with a weight ratio of1:1:1 were combined and made into a washcoat slurry via addition tode-ionized water, milling to an appropriate particle size (typicallywith a d50 range from 3 to 7 μm), and pH adjustment to give anappropriate viscosity for washcoating. According to methods known in theart, the zeolite washcoat slurry was coated onto the cordierite monolith(with the first layer of PtPd), dried at 120° C. and calcinated at 500°C. to give the second layer of the multi-layer coated monolith. Eachzeolite washcoat loading is 0.15 g/in³.

Then, the supported PtPd catalyst powder (4.0% Pt, 2.0% Pd) prepared asdescribed above was made into a washcoat slurry via addition tode-ionized water, milling to an appropriate particle size (typicallywith a d50 range from 3 to 7 μm), and pH adjustment to give anappropriate viscosity for washcoating. According to methods known in theart, the washcoat slurry was coated onto the cordierite monolith (withthe first layer of PdAu and the second layer of zeolite mixtures), driedat 120° C. and calcined at 500° C. to give the third layer of themulti-layer coated monolith, such that the PtPd loading was ˜0.9 g/in³.

The multi-layer coated monolith was canned according to methods known inthe art and tested using a certified testing facility on a light-dutydiesel vehicle, as described above.

EXAMPLE 5 PdAu (at 1.0 g/in³) 1st Layer, Zeolite Mixture(ZSM/β=0.15/0.0.15 g/in³) 2nd Layer, PtPd (at 0.9 g/in³) 3rd Layer.

The supported PdAu catalyst powder (3.0% Pd, 3.6% Au) prepared asdescribed above was made into a washcoat slurry via addition tode-ionized water, milling to an appropriate particle size (typicallywith a d50 range from 3 to 7 μm), and pH adjustment to give anappropriate viscosity for washcoating. According to methods known in theart, the washcoat slurry was coated onto a round cordierite monolith(Corning, 350 cpsi, 48.16 in³), dried at 120° C. and calcined at 500° C.to give the first layer of the multi-layer coated monolith, such thatthe PdAu loading was ˜1.0 g/in³.

Then, ZSM-5 zeolite and beta zeolite with a weight ratio of 1:1 werecombined and made into a washcoat slurry via addition to de-ionizedwater, milling to an appropriate particle size (typically with a d50range from 3 to 7 μm), and pH adjustment to give an appropriateviscosity for washcoating. According to methods known in the art, thezeolite washcoat slurry was coated onto the cordierite monolith (withthe first layer of PtPd), dried at 120° C. and calcinated at 500° C. togive the second layer of the multi-layer coated monolith. Both ZSM-5zeolite and beta zeolite washcoat loading are 0.15 g/in³.

Then, the supported PtPd catalyst powder (4.0% Pt, 2.0% Pd) prepared asdescribed above was made into a washcoat slurry via addition tode-ionized water, milling to an appropriate particle size (typicallywith a d50 range from 3 to 7 μm), and pH adjustment to give anappropriate viscosity for washcoating. According to methods known in theart, the washcoat slurry was coated onto the cordierite monolith (withthe first layer of PdAu and the second layer of zeolite mixtures), driedat 120° C. and calcined at 500° C. to give the third layer of themulti-layer coated monolith, such that the PtPd loading was ˜0.9 g/in³.

The multi-layer coated monolith was canned according to methods known inthe art and tested using a certified testing facility on a light-dutydiesel vehicle, as described above.

While particular embodiments according to the invention have beenillustrated and described above, those skilled in the art understandthat the invention can take a variety of forms and embodiments withinthe scope of the appended claims.

1. An emission control catalyst comprising a substrate having first,second, and third washcoat layers coated thereon in a multi-layerconfiguration, wherein the first washcoat layer includes palladium andgold particles in close contact and the third washcoat layer includessupported precious group metal particles and the second washcoat layeris between the first washcoat layer and the third washcoat layer andcontains at least Y zeolite.
 2. The emission control catalyst accordingto claim 1, wherein the second washcoat layer contains no other types ofzeolite.
 3. The emission control catalyst according to claim 1, whereinthe second washcoat layer further contains ZSM-5 zeolite.
 4. Theemission control catalyst according to claim 3, wherein the weight ratioof the Y zeolite to the ZSM-5 zeolite is 2:1.
 5. The emission controlcatalyst according to claim 3, wherein the second washcoat layer furthercontains beta zeolite.
 6. The emission control catalyst according toclaim 3, wherein equal weights of the Y zeolite, the ZSM-5 zeolite, andthe beta zeolite are contained in the second washcoat layer.
 7. Theemission control catalyst according to claim 1, wherein the substratehas a honeycomb structure with gas flow channels, and the first, second,and third washcoat layers are coated on the walls of the gas flowchannels.
 8. The emission control catalyst according to claim 7, whereinthe first washcoat layer is disposed directly on the walls of the gasflow channels and the third washcoat layer is disposed to be in directcontact with exhaust gases flowing through the gas flow channels.
 9. Theemission control catalyst according to claim 8, wherein the firstwashcoat layer comprises supported palladium-gold metal particles, andthe second washcoat layer comprises a mixture of Y zeolite and ZSM-5zeolite, and the third washcoat layer comprises supportedplatinum-palladium metal particles.
 10. An emission control catalystcomprising: a substrate having first, second, and third washcoat layerscoated thereon in a multi-layer configuration, wherein the firstwashcoat layer comprises supported palladium-gold metal particles, andthe third washcoat layer comprises supported platinum-palladium metalparticles, and wherein the second washcoat layer contains at least Yzeolite and is between the first washcoat layer and the third washcoatlayer.
 11. The emission control catalyst according to claim 10, whereinthe first washcoat layer is disposed on the substrate, the secondwashcoat layer on the first washcoat layer, and the third washcoat layeron the second washcoat layer.
 12. The emission control catalystaccording to claim 10, wherein the second washcoat layer contains noother types of zeolite.
 13. The emission control catalyst according toclaim 12, wherein the second washcoat layer further contains ZSM-5zeolite.
 14. The emission control catalyst according to claim 13,wherein the weight ratio of the Y zeolite to the ZSM-5 zeolite is 2:1.15. The emission control catalyst according to claim 13, wherein thesecond washcoat layer further contains beta zeolite.
 16. The emissioncontrol catalyst according to claim 15, wherein equal weights of the Yzeolite, the ZSM-5 zeolite, and the beta zeolite are contained in thesecond washcoat layer.
 17. The emission control catalyst according toclaim 10, wherein the substrate has a honeycomb structure with gas flowchannels, and the first, second, and third washcoat layers are coated onthe walls of the gas flow channels.
 18. The emission control catalystaccording to claim 17, wherein the first washcoat layer is disposeddirectly on the walls of the gas flow channels and the third washcoatlayer is disposed to be in direct contact with exhaust gases flowingthrough the gas flow channels.
 19. The emission control catalystaccording to claim 10, wherein the supported palladium-gold metalparticles consists essentially of palladium and gold particles in closecontact.
 20. The emission control catalyst according to claim 1, whereinthe second washcoat layer further contains beta zeolite.
 21. Theemission control catalyst according to claim 10, wherein the secondwashcoat layer further contains beta zeolite.